There is a continuing need in the graphic arts and related industries to transfer images photographically from original (positive or negative) photomasks to light sensitive sheet elements to form a set of imaged elements which can be combined in register to produce a composite image. The set of imaged elements may be combined directly as for instance in the formation of an overlay picture from registered superposition of individual transparencies, or the use of the set of imaged elements may require further process steps as for instance the formation of a multicolored printed image using a set of individual photopolymer printing plates. In either case, each imaged element of a set must be provided with a common means of registration by which a final composite image can be prepared by aligning or registering precisely the locations previously established on each imaged element of the set. In all cases, the photographic operation is carried out in a manner which maintains, to the greatest precision possible, both the definition and the relative locations of the features of the images.
Composite images formed by simple assembly of the imaged elements in register are typified by the preparation of multicolor overlays and particularly four-color overlay proofs. Such processes are disclosed in U.S. Pat. Nos. 3,060,024; 4,282,308; and 4,229,520. In the processes described, each of four photopolymerizable elements comprising a photopolymer layer and a transparent support is imagewise exposed to a photomask representing each of the four colors to be used, e.g., black, yellow, magenta and cyan. Either the imaging exposure produces registration marks on the photopolymer element or both the photomask and the photohardenable element contain at least two contact points, usually precisely punched holes. After imaging exposure the elements are then processed to produce a set of imaged elements each having a different uniform color but all having the same registration marks or points. By superposing the four elements and aligning the marks or points, a full color rendition of the original picture is formed.
Composite images formed by sequential transfer in register of images from a set of imaged photosensitive elements to a receptor sheet is typified by full color printing using relief printing, planographic printing, and intaglio printing, or by formation of four color proofs by a transfer process similar to that described in U.S. Pat. Nos. 3,060,024 and 3,582,327. The fundamentals of full color printing are described in "The Printing Industry" by Victor Strauss; Printing Industries of America Inc. 1967, and photopolymer elements for preparing printing plates are described in U.S. Pat. Nos. 2,760,863; 2,791,504; 2,964,401; 3,458,311; 4,072,527, 4,072,528; 3,829,204; 4,323,637 and 4,177,074.
Contact printing is virtually the universal method of exposure used today in the previously described photoduplication processes despite certain known shortcomings. Although low in equipment costs, simple to use, and capable of excellent line and halftone definition, contact printing is labor intensive and slow (because of long conditioning and/or vacuum draw-down times). It also is subject to losses due to damaged or dirty photomasks resulting from repeated use. This, in turn, requires frequent and expensive touch up and replacement of photomasks to avoid yield penalties. Much time is also lost in the constant and tedious process of inspecting photomasks for detects between exposure. In addition, variations in frame temperature and ambient humidity affect corner-to-corner registration, especially for large elements such as printing plate.
Alternative exposure methods such as gap printing, projection printing and laser scanning each offer some significant advantages over contact printing. However in the current state of development, all have serious limitations for high productivity applications and are intrinsically much higher in equipment cost.